As a conductive material, a conductive paste, a conductive adhesive, an anisotropic conductive film and the like can be exemplified and conductive compositions containing conductive fine particles and resins are used for those conductive materials.
As such conductive fine particles, generally, metal powder, carbon powder, and fine particles plated with a metal plating layer on the surface, and the like have been used. Methods for producing conductive fine particles having metal plating layers on the surfaces are disclosed, for example, in Japanese Kokai Publication Sho-52-147797, Japanese Kokai Publication Sho-61-277104, Japanese Kokai Publication Sho-61-277105, Japanese Kokai Publication Sho-62-185749, Japanese Kokai Publication Sho-63-190204, Japanese Kokai Publication Hei-1-225776, Japanese Kokai Publication Hei-1-247501, Japanese Kokai Publication Hei-4-147513, and the like.
In these production methods, a method using a barrel plating apparatus has commonly been employed in the case of plating fine particles with a particle diameter of 5,000 μm or less. The barrel plating apparatus is for carrying out electric plating by putting an article to be plated in a rotatable polygonal or cylindrical barrel immersed in a plating solution and bringing the plated article into contact with a cathode installed in the barrel while rotating the barrel. However, the method for producing conductive fine particles using a conventional barrel plating apparatus has a problem that the fine particles to be plated are easy to be agglomerated to one another during the plating process.
On the contrary, for example, a method is proposed for forming a plating layer on a chip resistor element by loading a large number of power supplying bodies, which comprise conductive metal balls called as dummy, and a stirring promoter of ceramic balls or the like in a barrel. However, the method has a problem that an adhesion of chips to one another occur after plating and result in impossibility of separating them as independent chip parts.
Japanese Kokai Publication Hei-11-200097 proposes a barrel plating method for chip parts with considerably suppressed occurrence of the adhesion trouble of chip parts to one another by loading adjustment bodies with the same shape as that of non-conductive chip parts and a large number of metal power supplying bodies and then carrying out plating. However, according to the method, although the adhesion of chip parts can be suppressed, it is insufficient to suppress occurrence of agglomeration of fine particles when the method is applied for plating fine particles.
Meanwhile, conventionally, an alkaline cyanogen solution containing a cyanogen compound has been known as an electrolytic plating solution to be used for forming a tin/silver alloy plating layer. However, since the alkaline cyanogen solution contains a cyanogen compound, the solution has problems that it is very toxic and thus has to be handled extremely carefully; it requires particular wastewater treatment; and it worsens the work environments.
For these problems, Japanese Kokai Publication Hei-11-269692 proposes an acidic bath containing no cyanogen compound as a tin/silver alloy electrolytic plating solution and describes that it is possible to form a tin/silver alloy plating film excellent in brightness, solderability, and whisker property, using this acid bath. When electric plating is carried out using such a tin/silver alloy electrolytic plating solution, an object article to be plated is used as a cathode and a tin or an insoluble electrode is used as an anode.
However, in the case of electrically plating fine particles, the surface area of the fine particles becomes extremely wide to the quantity of the electrolytic plating solution and accordingly, the silver concentration in the electrolytic plating solution is decreased along with the proceeding of the plating and when the electric plating is continued, the tin/silver composition of the alloy differs in the thickness direction of the formed plating film and the ratio of the silver component decreases more as it goes outer and consequently, in an extreme case, it leads to a problem that the formed plating layer has an outermost layer of 100% tin.